A torque converter installed in a transmission of a vehicle is provided with a lockup clutch serving as a friction element. By setting the lockup clutch in an engaged condition, a deviation between input/output rotation speeds of the torque converter is suppressed, leading to an improvement in the fuel efficiency of an engine.
JP2004-138192A discloses a lockup clutch control method in which a slip rotation speed between a pump impeller and a turbine impeller in a torque converter is controlled by precharging a supplied fluid pressure to eliminate a dead zone of a piston and supplying further fluid pressure following the precharge to increase a pressing force of a piston has been disclosed as a control method for a lockup clutch of a torque converter.
In this conventional technique, the lockup clutch is engaged when an operating condition of the vehicle is within a lockup ON region and the lockup clutch is set in a disengaged condition when the operating condition of the vehicle is not in the lockup ON region.